The present invention relates to a rack-pinion type power steering apparatus and, more particularly, to the support structure of a pinion shaft integrally formed with a sleeve serving as the outer valve member of a rotary type channel selector valve (rotary valve).
A rack-pinion type power steering apparatus is advantageous in that it is lightweight, has a comparatively simple arrangement, is excellent in steering performance, and requires a small space when being incorporated in a vehicle, and is accordingly among those which are often employed as power steering. Although many apparatuses having various structures are conventionally proposed as such a rack-pinion type power steering apparatus, they still have room for improvement in achieving downsizing and weight reduction of an automobile and making the apparatus compact.
For example, each of Japanese Utility Model Laid-Open Nos. 55-114763 and 55-114764 discloses a structure in which a sleeve serving as an outer valve member constituting a rotary type channel selector valve is integrally formed on a pinion shaft having pinion teeth that mesh with the rack teeth of a rack in order to further decrease the size of the entire apparatus and to simplify the working and assembly operations.
In the former conventional case, a pinion shaft integrally having a sleeve is axially supported by a steering body with rolling bearings, e.g., a ball bearing and a needle bearing, formed on the two end portions of the pinion shaft. The valve housing portion can be integrally formed with the steering body.
In the latter conventional case, the two end portions of the pinion teeth of the pinion shaft are axially supported by a steering body with rolling bearings, e.g., a ball bearing and a needle bearing.
A conventional rack-pinion type power steering apparatus in which the pinion shaft and the sleeve serving as the outer valve member constituting the rotary type channel selector valve are integrally formed is known, as described above. In this conventional apparatus, however, when supporting the pinion shaft integrally formed with the sleeve, the two end portions of the pinion shaft including this sleeve in the steering body, or the two end portions of the pinion teeth of the pinion shaft are merely axially supported with the rolling bearings. With this arrangement, when a large load acts on the pinion shaft to deform it by bending, the sleeve serving as the outer valve member constituting the rotary type channel selector valve is also deformed by bending. Then, decentering, offset contact, and the like occur between the sleeve and the rotor serving as the inner valve member, thus interfering with a smooth valve action.
In particular, in this rotary type channel selector valve, the rotor integral with the stub shaft of the steering wheel and the sleeve integral with the pinion shaft of the steering wheel must be combined such that they can concentrically, rotatably displace relative to each other and be disposed in the valve housing. This is because the oil pump serving as the fluid pressure generating source, the oil tank, the inlet port and the return port communicating with the left and right cylinder chambers constituting the power cylinder, and the channels serving as the right and left output ports are connected to each other by selectively connecting and disconnecting a plurality of channel grooves radially formed in the outer circumferential portion of the rotor and the inner circumferential portion of the sleeve to and from each other by relative rotational displacement between the rotor and sleeve, thereby selecting the channels of the fluid-pressure circuit (hydraulic circuit).
In the conventional rack-pinion type power steering apparatus described above, when the sleeve is integrally formed on the pinion shaft, a smooth valve action is hindered due to the reason described above. A countermeasure that can solve this problem is therefore sought for.